A signal generator outputs a current signal having a predetermined waveform in accordance with electric load or forms and outputs a signal for communication (i.e., a communication signal) on the basis of a direct current power supply. For example, when the electric load such as a lamp and a coil mounted in an automotive vehicle is operated or stopped operating, a noise is generated by rapid current change at the moment when electricity is turned on or off. The noise may harm a radio and a control circuit mounted on the vehicle. Specifically, the impedance of the electric load is changed by heat together with energization. The impedance is low just after the electricity starts to be supplied to the electric load. At this time, the current change is greater than that of a case when the electricity is energized constantly. To reduce the noise, a signal generator is disclosed in U.S. Pat. No. 6,545,513-B2. In the signal generator, current waveform during PWM control is set to be a trapezoid form so that current change at the moment of energization or de-energization becomes moderate. Thus, the noise level is reduced by the signal generator.
To reduce the noise level, it is preferred that a slope at a rising edge and a down edge of the current waveform becomes smaller. However, when the slope becomes small, a switching device such as a FET may generate large heat because a switching loss in the device increases. Thus, it is required to select the switching device having sufficient thermal resistance for withstanding the large heat and/or to prepare a heat sink for radiating the large heat. Therefore, a manufacturing cost is increased, and design freedom of the signal generator is reduced. Further, it is required to reduce noise in the switching device not only for the electric load but also for a communication by slowing a slope at a rising edge and a down edge in a current waveform.